Electric Power Systems Research 77 (2007) 1552–1559 Adaptive relays for overhead line protection V. Calderaro, V. Galdi, A. Piccolo ∗ , P. Siano Department of Information and Electrical Engineering (DIIIE), University of Salerno, via Ponte don Melillo, 1, 84084 Fisciano, Salerno, Italy Received 14 March 2006; received in revised form 18 September 2006 Available online 8 December 2006 Abstract In the liberalized energy market scenario protective relays play an important role in assuring continuous service in the power system where a malfunctioning could lead to serious damages to a wide number of operators having access to the power system. Considering that power lines are operated many times below a rated load current, in this paper an adaptive procedure is presented in order to manage power distribution systems according to dependability or security requirements. In particular, a procedure to obtain an inverse time trip curve by means of a microprocessor, connected to a relay, is presented. The procedure adapts the trip characteristic depending on the conductor temperature, wind speed, emissivity and solar absorbity and is implemented on a microprocessor Rabbit 2200 considering a Drake conductor, 795 kcmil 26/7 ACSR. © 2006 Published by Elsevier B.V. Keywords: Protection system; Overhead line; Adaptive relay 1. Introduction The reliability and efficiency of power systems depend on the use of the automation in transmission, substation and distribution systems, which is growing fast and is becoming increasingly handy with the rapid development of the modern communication technology [1]. The automation control system must be able to manage a wide variety of emerging intelligent electronic devices (IEDs) including new protection systems, power electronic equipment for power quality management and smart capacitor banks. In par- ticular, there have been significant advances in the field of pro- tective relaying due to both the impact of computers and micro- processors and the introduction of the adaptive relaying concept. Protective relays play an important role in the liberalized energy market scenario since they assure continuous service in the power system where a malfunctioning could lead to serious damages to a wide number of operators having access to the power system. Traditionally, in distribution systems, transformers, cables and overhead lines have been operated below a rated load ∗ Corresponding author. Tel.: +39 089 964210/84; fax: +39 089 964210/84. E-mail addresses: vcalderaro@unisa.it (V. Calderaro), vgaldi@unisa.it (V. Galdi), piccolo@unisa.it (A. Piccolo), psiano@unisa.it (P. Siano). current. However, the rating values, provided by equipment manufactures, are set by regulations based on particular criti- cal conditions. Nevertheless ambient and operative conditions are often not as severe as the worst case, therefore, equipments can handle higher load currents, while remaining within their design parameters [2]. In order to allow a longer continuous service, during overload conditions, relays can be used to protect equipments by adapting their characteristics to ambient and technical conditions. Several studies in literature deal with adaptive relays, focus- ing the attention on the application aspects or on the adaptation methodology. In particular in [3] the objective is the investiga- tion of the effect of mid-point compensation of STATCOM on the performance of impedance distance relay under normal load and fault conditions and the implementation of an adaptive dis- tance relaying scheme for transmission line protection. In [4,5] a Multi Agent System based on adaptive protection relay system is presented. In [4] the architecture and the function model of the system are proposed and the validity of the proposed scheme is verified by means of simulations. In [5] a cooperative protection system, which processes local data in order to achieve adap- tive protection, is presented. The effect of distributed generation (DG) on the coordination protection is explored in [6] where a high DG penetration on distribution system suggests an adaptive protection scheme as a solution to some identified problems. As stated by many authors, overhead lines are operated below a rated load current and several papers evaluate the lines thermal 0378-7796/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.epsr.2006.11.001